Medical practitioners have found it useful to use robotic systems to assist in the performance of surgical procedures. Such robotic systems typically include a manipulator having a moveable arm comprising one or more links. A surgical instrument is attached to a free end of the arm. The instrument is designed to be applied to a surgical site. A controller regulates movement of the arm to position the instrument with a high degree of accuracy at the surgical site.
A component of many robotic systems is a force/torque sensor assembly. The force/torque sensor assembly is attached between the free end of the arm and the instrument. The force/torque sensor assembly monitors forces and torques that are applied to the instrument. These may be forces and torques that are applied to the instrument as a consequence of the instrument pressing against tissue. These also may be forces and torques a practitioner applies in order to set a position and/or orientation of the instrument. Signals output by the force/torque sensor assembly are received by the controller. The controller uses these signals to determine a target position for the instrument. Based on the determined target position, the controller actuates the arm in order to advance the arm so that the instrument is moved to the target position.
In order to ensure all forces and torques applied to the instrument are measured, it is common practice to provide a six component force/torque sensor assembly. This type of force/torque sensor assembly measures forces applied to the instrument along three axes and torques applied to the instrument around the three axes.
One type of six component force/torque sensor assembly comprises a set of strain gauges. These gauges include a static member to which a plurality of beams are flexibly mounted. Typically one or more strain gauges are associated with each beam. Each strain gauge acts as a transducer that is used to convert a force or torque into an electrical signal. Each strain gauge generates an electrical signal proportional to the flexure of the beam with which the strain gauge is associated. The output signals from the strain gauges are input variables into an algorithm that yields the measured forces and torques.
In force/torque sensor assemblies employing strain gauges, thermal drift is a common problem. Thermal drift occurs when a change in temperature causes a contraction or expansion of parts. Thermal drift can result in inaccurate placement of the instrument at the surgical site. It is also common for these types of force/torque sensor assemblies to take force and torque measurements at a single resolution. In some cases it may be desirable for the force/torque sensor assembly to be capable of measuring forces and torques at multiple resolutions.
Thus, there is a need in the art for a force/torque sensor assembly and method of measuring forces and torques that overcomes one or more of these deficiencies.